Image forming apparatus that transfers toner image carried by image carrier onto sheet, density control method therefor, and storage medium

ABSTRACT

An image forming apparatus capable of identifying the amount of reflected light from any portion of an image carrier in a short time. A sensor detects reflected light from an intermediate transfer belt. A CPU performs pattern matching between the amount of reflected light from the belt corresponding to one rotation thereof and the amount of reflected light from a specific portion of the belt to thereby identify a first circumferential location of the specific portion and a second circumferential location of a toner patch formed on the belt. The CPU calculates toner patch density based on the amount of reflected light from the toner patch and the amount of reflected light from the belt in the second circumferential location. The density of a toner image to be formed on the belt is controlled according to the calculated toner patch density.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus adapted totransfer a toner image carried by an image carrier onto a sheet, and adensity control method for the image forming apparatus.

2. Description of the Related Art

Conventionally, in an image forming apparatus, such as a copying machineor a printer utilizing electrophotography, the density of an image hasbeen corrected in the following manner: A toner image is formed fordensity correction (i.e. a toner patch) on an image carrier, such as aphotosensitive drum or an intermediate transfer member, and the tonerpatch is detected by an optical sensor to thereby generate correctiondata.

In the case of determining the density of a toner patch, it is requiredto grasp the amount of reflected light from a portion of the imagecarrier reflected light from a portion of the image carrier where thetoner patch is to be formed, i.e. from a so-called background, inadvance. This is because an output from the sensor having detected thetoner patch also contains reflected light from the background.

The photosensitive drum and the intermediate transfer member are glossy,so that much of light irradiated onto the photosensitive drum or theintermediate transfer member is reflected therefrom to be read by theoptical sensor. In particular, a low-density image is expressed byreducing the amount of toner, and hence the degree of exposure of abackground on which a low-density toner patch is formed is higher thanthat of a background on which a high-density toner patch is formed. Forthis reason, in order to calculate the density of a low-density tonerpatch accurately, it is required to detect the density of the tonerpatch while taking into account the amount of reflected light from itsbackground.

Conventionally, there has been proposed a method in which a homeposition mark provided on an image carrier is detected by an opticalsensor to thereby obtain the positional relationship between the homeposition of the image carrier for a rotation thereof and a toner patch,and then the amount of reflected light from the background of the tonerpatch is identified based on the positional relationship (see JapaneseLaid-Open Patent Publication No. 2005-345740).

In this method, surface conditions of the image carrier during onerotation of the same are detected as a profile in advance. Further, anoutput indicative of reflected light from the background of the tonerpatch is identified based on the positional relationship between thehome position and the toner patch and the profile of the surfaceconditions of the image carrier detected in advance over one rotation ofthe image carrier, and the density of the toner patch is detected basedon the identified output indicative of reflected light from thebackground and the result of detection of the toner patch.

Further, conventionally, there has been proposed an apparatus which doesnot use the above-mentioned home position mark (see Japanese Laid-OpenPatent Publication No. 2005-148299). In this apparatus, background datacorresponding to one rotation of an intermediate transfer member ismeasured, and then image density detection data corresponding to onerotation of the intermediate transfer member having a toner patch formedthereon is measured. Thereafter, alignment between the background dataand the image density detection data is performed based on a correlationbetween the two data. Thus, background data on a portion of theintermediate transfer member where the toner patch is formed isidentified based on the result of the alignment.

However, the conventional image forming apparatuses described abovesuffer from the following problems: In the image forming apparatusdisclosed in Japanese Laid-Open Patent Publication No. 2005-345740, ifthe home position mark is lost due to fall-off or abrasion, it becomesimpossible to perform density correction by taking reflected light fromthe background into account. Further, it takes cost to attach the homeposition mark.

On the other hand, in the image forming apparatus disclosed in JapaneseLaid-Open Patent Publication No. 2005-148299, after acquisition of thebackground data, it is required to cause the intermediate transfermember to perform one more rotation with the toner patch formed thereon,so as to acquire data for density correction, and therefore it takestime to perform density correction.

SUMMARY OF THE INVENTION

The present invention provides an image forming apparatus which iscapable of identifying the amount of reflected light from a desiredportion of an image carrier in a short time by a simplifiedconstruction, and a density control method for the image formingapparatus.

In a first aspect of the present invention, there is provided an imageforming apparatus that transfers a toner image carried by an imagecarrier onto a sheet, comprising a photo detector unit disposed infacing relation to the image carrier, for detecting reflected light fromthe image carrier, a circumferential location identification unitadapted to perform pattern matching between amounts of reflected lightdetected by the photo detector unit during one rotation of the imagecarrier and an amount of reflected light detected by the photo detectorunit from a specific portion of the image carrier in a circumferentialdirection of the image carrier, to thereby identify a firstcircumferential location of the specific portion of the image carrier inthe circumferential direction of the image carrier and then identify asecond circumferential location of a detection toner image formed on theimage carrier based on the identified first circumferential location, adensity calculation unit adapted to calculate density of the detectiontoner image based on an amount of reflected light from the detectiontoner image, which is detected by the photo detector unit, and an amountof reflected light from a portion of the image carrier in the secondcircumferential location identified by the circumferential locationidentification unit, out of amounts of reflected light from the imagecarrier corresponding to one rotation of the intermediate transfer belt,which are detected by the photo detector unit, and a density controlunit adapted to control density of a toner image to be formed on theimage carrier, according to the density of the detection toner imagecalculated by said density calculation unit.

The image forming apparatus according to the first aspect of the presentinvention performs pattern matching between the amount of reflectedlight from the specific portion of the image carrier and the amounts ofreflected light from the image carrier corresponding to one rotationthereof to thereby identify the first circumferential location, and thenidentifies the second circumferential location of the detection tonerimage based on the identified first circumferential location. Thus, theimage forming apparatus can identify the amount of reflected light fromany portion of the image carrier in a short time with the simplifiedconstruction, which makes it possible to easily acquire the amount ofreflected light from a portion of the image carrier to be used as abackground for the detection toner image.

In a second aspect of the present invention, there is provided a densitycontrol method for an image forming apparatus that is adapted totransfer a toner image carried by an image carrier onto a sheet, andincludes a photo detector unit disposed in facing relation to the imagecarrier, comprising detecting reflected light from the image carrier bythe photo detector unit, performing pattern matching between amounts ofreflected light detected by the photo detector unit during one rotationof the image carrier and an amount of reflected light detected by thephoto detector unit from a specific portion of the image carrier in acircumferential direction of the image carrier, to thereby identify afirst circumferential location of the specific portion of the imagecarrier in the circumferential direction of the image carrier and thenidentify a second circumferential location of a detection toner imageformed on the image carrier based on the identified firstcircumferential location, calculating density of the detection tonerimage based on an amount of reflected light from the detection tonerimage, which is detected by the photo detector unit, and an amount ofreflected light from a portion of the image carrier in the identifiedsecond circumferential location, out of amounts of reflected light fromthe image carrier corresponding to one rotation of the intermediatetransfer belt, which are detected by the photo detector unit, andcontrolling density of a toner image to be formed on the image carrier,according to the calculated density of the detection toner image.

According to the density control method of the second aspect of thepresent invention, it is possible to obtain the same advantageous effectas provided in the first aspect.

In a third aspect of the present invention, there is provided anon-transitory computer-readable storage medium storing a program which,on execution by a programmable image forming apparatus that is adaptedto transfer a toner image carried by an image carrier onto a sheet, andincludes a photo detector unit disposed in facing relation to the imagecarrier, causes the programmable image forming apparatus to carry out adensity control method comprising detecting reflected light from theimage carrier by the photo detector unit, performing pattern matchingbetween amounts of reflected light detected by the photo detector unitduring one rotation of the image carrier and an amount of reflectedlight detected by the photo detector unit from a specific portion of theimage carrier in a circumferential direction of the image carrier, tothereby identify a first circumferential location of the specificportion of the image carrier in the circumferential direction of theimage carrier and then identify a second circumferential location of adetection toner image formed on the image carrier based on theidentified first circumferential location, calculating density of thedetection toner image based on an amount of reflected light from thedetection toner image, which is detected by the photo detector unit, andan amount of reflected light from a portion of the image carrier in theidentified second circumferential location, out of amounts of reflectedlight from the image carrier corresponding to one rotation of theintermediate transfer belt, which are detected by the photo detectorunit, and controlling density of a toner image to be formed on the imagecarrier, according to the calculated density of the detection tonerimage.

According to the non-transitory computer-readable storage medium of thethird aspect of the present invention, it is possible to obtain the sameadvantageous effect as provided in the first aspect.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments (with reference to theattached drawings).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of image forming units of an image formingapparatus according to a first embodiment of the present invention.

FIG. 2 is a view of toner patches formed on an intermediate transferbelt.

FIG. 3 is a view of the intermediate transfer belt having the tonerpatches and page images formed thereon.

FIG. 4 is a view showing the arrangement of a sensor.

FIG. 5 is a graph showing reflected light amount distributions eachcorresponding to one rotation of the intermediate transfer belt, betweenwhich the amount of emitted light is changed.

FIG. 6 is a graph showing the relationship between toner patch densityand the amount of reflected light.

FIG. 7 is a block diagram of an image processing unit of the imageforming apparatus.

FIG. 8A is a diagram showing a table of a one-rotation backgroundprofile.

FIG. 8B is a graph of the one-rotation background profile.

FIG. 9 is a graph showing specular reflection light output obtainedduring a time period from the start of reading of a partial backgroundprofile to the start of reading of the toner patches.

FIG. 10A is a diagram showing a table of a reference profile.

FIG. 10B is a graph of the reference profile.

FIG. 11 is a graph showing a reflected light amount distributioncorresponding to one rotation of the intermediate transfer belt and areflected light amount distribution in a state where the intermediatetransfer belt has toner patches formed thereon, with their phasesaligned.

FIG. 12 is a graph showing a one-dimensional LUT stored in a RAM.

FIG. 13 is a flowchart of an image density control process.

FIG. 14 is a continuation of FIG. 13.

FIG. 15A is a graph of surface gloss representing the surface conditionsof the intermediate transfer belt which vary with the cumulative numberof printed sheets.

FIG. 15B is a graph of sensor output representing the surface conditionsof the intermediate transfer belt which vary with the cumulative numberof printed sheets.

FIG. 16A is a view showing conventional pattern matching.

FIG. 16B is a view showing pattern matching in the first embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present invention will now be described in detail below withreference to the accompanying drawings showing embodiments thereof.

FIG. 1 is a schematic view of image forming units of an image formingapparatus according to a first embodiment of the present invention. Thisimage forming apparatus is, for example, implemented by anelectrophotographic color image forming apparatus (printer) whichemploys an intermediate transfer belt 27 (image carrier) and has tandemimage forming units 10 of four colors, i.e. yellow, magenta, cyan, andblack.

Each laser beam source 24 emits a laser beam based on a digital signalfrom a document reader (not shown) to form an electrostatic latent imageon an associated photosensitive drum 22 uniformly charged by anassociated primary electrostatic charger 23. The tandem color imageforming apparatus of the present embodiment is provided with ayellow-associated laser beam source 24Y, a magenta-associated laser beamsource 24M, a cyan-associated laser beam source 24C, and ablack-associated laser beam source 24K associated with the respectivecolors. Similarly, the tandem color image forming apparatus is providedwith a yellow-associated photosensitive drum 22Y, a magenta-associatedphotosensitive drum 22M, a cyan-associated photosensitive drum 22C, anda black-associated photosensitive drum 22K, which are associated withthe respective colors. It should be noted that the laser beam sources24Y to 24K and the photosensitive drums 22Y to 22K are genericallyreferred to as the laser beam source 24 and the photosensitive drum 22,respectively, when it is not particularly required to differentiatebetween the laser beam sources and the photosensitive drums, based onthe colors.

The photosensitive drum 22 is formed by coating the outer periphery ofan aluminum cylinder with an organic light conductive layer, and isconfigured to perform rotation when a driving force is transmitted froma drive motor (not shown). The drive motor causes counterclockwiserotation of the photosensitive drum according to image formingoperation.

An electrostatic latent image formed on the photosensitive drum 22 isvisualized as a toner image by an associated one of developing devices26. The developing devices 26, i.e. four developing devices 26Y, 26M,26C, and 26K for developing yellow (Y), magenta (M), cyan (C), and black(K) toner images are provided at respective stations. The developingdevices 26Y, 26M, 26C, and 26K are provided with respective sleeves26YS, 26MS, 26CS, and 26KS.

The toner images formed on the respective photosensitive drums 22 aretransferred onto the intermediate transfer belt 27. The intermediatetransfer belt 27 rotates clockwise in synchronism with rotation of eachof the photosensitive drums 22Y, 22M, 22C, and 22K. The intermediatetransfer belt 27 is held in contact with the photosensitive drums 22Y,22M, 22C, and 22K, and the toner images formed on the photosensitivedrums 22Y, 22M, 22C, and 22K are primarily transferred onto theintermediate transfer belt 27 at the respective contact positions.

In the present embodiment, the intermediate transfer belt 27 isimplemented by a single-layer polyimide resin belt having acircumferential length of 895 mm. Further, carbon particulates in anappropriate amount are dispersed in the resin for adjustment of beltresistance. For this reason, the intermediate transfer belt 27 has ablack surface with high smoothness and glossiness. The rotational speedof the intermediate transfer belt 27 is set to 246 mm/sec which the sameas process speed.

The toner image carried on the intermediate transfer belt 27 istransferred by a transfer unit 28 onto a recording material 21, i.e. asheet conveyed from a sheet feeder 11. More specifically, the multicolortoner image on the intermediate transfer belt 27 is transferred onto therecording material 21 being conveyed forward in a state nipped betweenthe intermediate transfer belt 27 and a roller of the transfer unit 28.The toner image transferred onto the recording material 21 is heated andfixed by a heating roller 31 and a pressure roller 32 in a fixing unit30. The recording material 21 having the toner image fixed thereon isconveyed from the fixing unit 30 and is detected by a sheet dischargesensor 42, followed by being discharged.

Next, a description will be given of density images (hereinafterreferred to as “toner patches P”) formed for density correction and anoptical sensor 41 (hereinafter simply referred to as “the sensor 41”).The sensor 41 (photo detector unit) is disposed in facing relation tothe intermediate transfer belt 27 to detect the surface conditions ofthe intermediate transfer belt 27 and the toner patches P.

FIG. 2 is a view of the toner patches P formed on the intermediatetransfer belt 27. FIG. 3 is a view of the intermediate transfer belt 27having the toner patches P and page images formed thereon. The tonerpatches P are formed on an image carrier, such as a photosensitive drumor an intermediate transfer belt. In the present embodiment, the tonerpatches P (detection toner images) are formed on the intermediatetransfer belt 27.

An arrow in FIG. 2 shows the direction of rotation of the intermediatetransfer belt 27. Each toner patch P has a 25 mm-square shape, andthirty-two toner patches P in total are formed on the intermediatetransfer belt in a manner arranged in the direction of rotation(circumferential direction) such that eight patches having eightdifferent image printing ratios (density gradations), respectively, areprovided for each color of Y, M, C, and K.

The relationship between each toner patch and the printing ratio(density gradation) is set as follows:

Y1, M1, C1, K1=12.5%

Y2, M2, C2, K2=25%

Y3, M3, C3, K3=37.5%

Y4, M4, C4, K4=50%

Y5, M5, C5, K5=62.5%

Y6, M6, C6, K6=75%

Y7, M7, C7, K7=87.5%

Y8, M8, C8, K8=100%

In the present embodiment, the toner patches P are formed rearward of a100-th page image J100 and detected by the sensor 41. The sensor 41 isdisposed downstream of a primary transfer section (see FIG. 1) so as todetect the surface conditions of the intermediate transfer belt 27 andthe toner patches P formed on the intermediate transfer belt 27.

FIG. 4 is a view showing the arrangement of the sensor 41. The sensor 41is comprised of a light emitter 411 implemented e.g. by an LED, a lightreceiver 412 implemented e.g. by a photodiode, and an IC 413 thatcontrols the amount of light to be emitted by the light emitter 411.

The light emitter 411 is disposed at an angle of 45 degrees with respectto a normal to the intermediate transfer belt 27, for emitting light tothe intermediate transfer belt 27. The light receiver 412 is disposed ata location symmetrical to the light emitter 411 with respect to thenormal to the intermediate transfer belt 27, for receiving specularreflection light from the toner patches P. FIG. 4 shows a toner patch Ppassing a detection area for the sensor 41.

The IC 413 controls the amount of light emitted by the light emitter411, by adjusting a voltage applied to the light emitter 411 of thesensor 41. FIG. 5 is a graph showing reflected light amountdistributions each corresponding to one rotation of the intermediatetransfer belt, between which the amount of emitted light is changed. Inthe graph, the horizontal axis represents the circumferential location(phase) of the intermediate transfer belt, while the vertical axisrepresents the amount of reflected light. Further, a bold solid line “a”indicates a case where the amount of reflected light is large, while athin solid line “b” indicates a case where the amount of reflected lightis small. As shown in FIG. 5, when the amount of light emitted differs,the amount of reflected light from the same object also differs. Morespecifically, the stronger the emitted light, the larger the amount ofreflected light from the object is.

The IC 413 causes the sensor 41 to operate at two light amount levels.One of the two light amount levels is set as a level suitable fordetection of toner patch density. The other is set as a level suitablefor pattern matching, described hereinafter.

The level suitable for detection of toner patch density can be explainedas follows: FIG. 6 is a graph showing the relationship between the tonerpatch density and the reflected light amount. As shown in FIG. 6,high-density toner patches tend to be less responsive in respect of thereflected light amount therefrom to a change in toner patch density asthe light amount is larger. On the other hand, low-density toner patchestend to be progressively lower in the absolute value of the amount ofreflected light therefrom, as the light amount is smaller, which makesit difficult to distinguish the reflected light from uneven gloss of thebackground surface. The term “high density” in the present embodiment isintended to mean a density which is not lower than an optical density of1.0.

Therefore, it is desirable that a light amount level is maintained, asthe appropriate light amount level for detection of toner patch density,at a level which makes it possible to distinguish the amount ofreflected light from a low-density toner patch from uneven gloss of thebackground surface and at which the amount of reflected light from ahigh-density toner patch is highly responsive to a change in toner patchdensity.

In the present embodiment, a light amount level is adopted at which anaverage reflected light amount from the background surface correspondingto one rotation of the intermediate transfer belt is equal to 3.5[V]±0.1 [V]. Hereafter, this light amount level will be referred to as“the patch-detecting light amount level”.

On the other hand, the appropriate level for pattern matching can bedefined as a light amount level which maximizes a rise or fall in thevalue of the reflected light amount from the background surface. In acase where pattern matching, described hereinafter, is performed betweenthe amount of reflected light from a portion of the background surfaceand the amount of reflected light from the background surfacecorresponding to one rotation of the intermediate transfer belt,accuracy in pattern matching is improved when the rise or fall in thevalue of the reflected light amount is larger.

FIG. 5 shows a reflected light amount distribution in a case where therise or fall in the value of the reflected light amount is large and onein a case where the rise or fall in the value is small. As shown in FIG.5, when the reflected light amount is small (i.e. in a case indicated by“b”), the rise or fall in the value is also small, whereas when thereflected light amount is large (i.e. in a case indicated by “a”), therise or fall in the value is also large. In the present embodiment, alight amount level is adopted at which an average reflected light amountfrom the background surface corresponding to one rotation of theintermediate transfer belt is equal to 4.5 [V]±0.1 [V]. Hereafter, thislight amount level will be referred to as “the pattern-matching lightamount level”.

Although in the present embodiment, the amount of light emitted from thelight emitter 411 is adjusted to thereby obtain the appropriatereflected light amount for pattern matching, some other method may beemployed to obtain the appropriate reflected light amount for patternmatching. More specifically, it is possible to employ a method in whichthe output gain of the light receiver 412 is adjusted using a variableresistor or a method in which both the output gain of the light emitter411 and that of the light receiver 412 are adjusted.

FIG. 7 is a block diagram of an image processing unit 50 of the imageforming apparatus. A CPU 51 performs centralized overall control ofcomponent elements of the image processing apparatus based on controlprograms stored in a ROM 52, using a RAM 53 as a work memory.

The RAM 53 stores a one-rotation background profile representingreflected light from the surface of the background of the toner patchesP corresponding to the one rotation of the intermediate transfer belt,which is read by the sensor 41. FIGS. 8A and 8B are diagrams of theone-rotation background profile. FIG. 8A is a diagram showing a table ofa one-rotation background profile and FIG. 8B is a graph showing theone-rotation background profile. The vertical axis of this graphrepresents the sensor output of the sensor 41, and the horizontal axisrepresents a detecting location (data number n) on the background.

In the present embodiment, immediately after the power of the printer isturned on, the intermediate transfer belt 27 rotates with no tonercarried thereon. At this time, the sensor 41 reads the surface of therotating intermediate transfer belt 27 corresponding to one rotation ofthe intermediate transfer belt. Specular reflection light output (sensoroutput) obtained by the scanning is stored as the one-rotationbackground profile (hereinafter simply referred to as “the one-rotationprofile”).

It should be noted that two kinds of one-rotation profiles are stored.One of them is a first one-rotation profile obtained by controlling thesensor 41 at the aforementioned patch-detecting light amount level andstored in the RAM 53, and the other is a second one-rotation profileobtained by controlling the sensor 41 at the aforementionedpattern-matching light amount level and stored in the RAM 53.

During the first rotation of the intermediate transfer belt 27, thesensor 41 is controlled to operate at the patch-detecting light amountlevel, and the first one-rotation profile is stored. During the secondrotation of the intermediate transfer belt 27, the sensor 41 iscontrolled to operate at the pattern-matching light amount level, andthe second one-rotation profile is stored.

FIG. 9 is a graph showing specular reflection light output obtainedduring a time period from the start of reading of a partial backgroundprofile until after the reading of a toner patch P. In the graph, thevertical axis represents sensor output from the sensor 41, and thehorizontal axis represents the data number n.

As shown in FIG. 9, a timer 55 measures a time period Tsec from thestart of reading of the partial background profile to the start ofreading of the toner patch P. This operation will be described in detailhereinafter.

The CPU 51 calculates a density DENS(i) of a toner patch P usingspecular reflection light output P(i) from the toner patch P(hereinafter referred to as “the toner patch reflection light outputP(i)”) and specular reflection light output R(i) from a portion of theintermediate transfer belt 27 immediately under the toner patch P(hereinafter referred to as “the toner patch background reflection lightoutput R(i)”), which is obtained during detection of the surfaceconditions of the intermediate transfer belt 27.

Further, the CPU 51 causes the sensor 41 to read an exposed portion ofthe surface of the intermediate transfer belt 27 between two images ofrespective pages (page images) formed in succession before formation oftoner patches P. Specular reflection light output (sensor output)obtained by the reading is stored as a partial background profile(hereinafter referred to as “the reference profile”). At this time, thesensor 41 is being controlled to operate at the aforementionedpattern-matching light amount level.

The CPU 51 performs pattern matching between the reference profileobtained by the reading and the second one-rotation profile to therebyidentify a portion of the second one-rotation profile which matches withthe reference profile, or an output portion closely analogous to thereference profile (see a frame f enclosed by dotted lines in FIG. 9). Itshould be noted that the sensor output level of the reference profile isequal to that of the second one-rotation profile, but in the graph inFIG. 9, these patterns are distinguishably illustrated for ease ofunderstanding.

The CPU 51 determines the toner patch background reflection light outputR(i) at the time of forming the toner patch P, based on the positionalrelationship on the intermediate transfer belt 27 between the identifiedportion and a location at which the toner patch P is formed.

The CPU 51 detects the density of the toner patch P and generatescorrection data based on the detected density.

As described above, the density of the toner patch P is calculated basedon the toner patch reflection light output (reflected light amount) P(i)and the toner patch background reflection light output (reflected lightamount) R(i). Further, correction data is generated based on thecalculated density. This process will be described in detailhereinafter. Then, the generated correction data is transmitted to theimage processing unit 50, described below, by a toner patch densitytransmission section incorporated in the CPU 51.

Next, a description will be given of the operation of the imageprocessing unit 50 that processes images read by a document reader. ACCD sensor 501 is provided in the document reader implemented e.g. by ascanner as an image reading device, and converts a read original imageto electric signals. The CCD sensor 501 is an RGB 3-line color sensor.Image signals of R (red), G (green), and B (blue) colors output from theCCD sensor 501 are input to an A/D converter unit 502.

The A/D converter unit 502 performs gain adjustment and offsetadjustment of the image signals, and then converts the image signals todigital image data of 8 bits on a color signal-by-color signal basis. Ashading correction unit 503 corrects variation in the sensitivity ofeach pixel of the CCD sensor 501, variation in the amount of light froman original-illuminating lamp, and so forth, on a color-by-color basis,using a read signal generated by reading a reference white board.

An input gamma correction unit 504 is a one-dimensional lookup table(LUT) that corrects each of input R, G, and B image data items such thatthe exposure amount of each color and luminance thereof satisfy a linearrelationship.

An input direct mapping unit 505 is a three-dimensional LUT thatconverts the input RGB signals to in-device RGB signals so as to form aunified color space. The three-dimensional LUT is provided to convert areading color space determined by the spectral characteristics of the R,G, and B filters of the CCD sensor 501 to a standard color space, suchas an sRGB (standard RGB), and is also capable of accommodatingcharacteristics, such as the sensitivity characteristics of the CCDsensor 501 and the spectral characteristics of the illuminating lamp.

A BE (Background Erase) sampling unit 506 discretely samples pixels in adesignated rectangular area so as to detect a background of an original,and forms a histogram of the luminance of the pixels. This histogram isused to erase the background during print processing.

A background erasing unit 507 performs nonlinear conversion for erasinga background portion on the RGB image data read by the scanner, based onthe results of sampling performed by the BE sampling unit 506. Then, theRGB image data is converted to CMYK image data by an output directmapping unit 508. To perform this conversion, the output direct mappingunit 508 inputs the values of the respective RGB colors to a lookuptable, and generates a C (cyan) component based on the total sum of theoutput values from the lookup table. Similarly, the output directmapping unit 508 generates the respective components of M (magenta), Y(yellow), and K (black) using lookup tables and performing additionoperations of the output values from the lookup tables.

An output gamma correction unit 509 performs density correction suchthat an output image becomes compatible with the printer. The outputgamma correction unit 509 plays the role of maintaining linearity ofoutput image data, which varies with every image formation, based on aone-dimensional lookup table of CMYK stored in advance.

The optical sensor 41 associated with density detection, the RAM 53, andthe CPU 51 creates the one-dimensional lookup table of CMYK. Theone-dimensional lookup table of CMYK is updated in timing in which thetoner patch density transmission section sends a one-dimensional LUTcreated anew to the output gamma correction unit 509. It should be notedthat a process executed by the CPU 51 will be described in detailhereinafter with reference to a flowchart.

A halftone processing unit 510 can selectively apply a different type ofscreening according to a function of the apparatus. In general, thehalftone processing unit 510 uses an error-diffusion type screeningwhich can suppress moire, for a copying operation, and a multi-valuedscreen type screening using a dither matrix because of excellentreproducibility of text data and thin lines, for a printing operation.

The former screening is a method which assigns weights to a target pixeland peripheral pixels using error filters, to thereby distributemultivalue conversion errors while maintaining the number of gradations,for correction of the errors. On the other hand, the latter is a methodwhich sets multi-valued thresholds of a dither matrix to thereby expresspseudo intermediate gradations. In the present embodiment, conversion isperformed independently for each of CMYKG, while switching between asmall line number (low dot density) and a large line number (high dotdensity) according to input image data, for reproduction.

Now, a description will be given of a toner patch density correctingmethod executed by the image forming apparatus of the presentembodiment. The toner patch density correcting method is executedfollowing steps (a) to (d) described below.

(a) The CPU 51 causes the sensor 41 to detect the intermediate transferbelt 27 during one rotation of the intermediate transfer belt after thepower is turned on. Further, immediately before the number of printedsheets reaches a predetermined number, the CPU 51 causes the sensor 41to detect a portion of the intermediate transfer belt 27 between twopage images printed in succession, while controlling the sensor 41 atthe pattern matching LED light amount level.

Then, after the predetermined number of sheets are printing operated,the CPU 51 causes toner patches P to be formed on the intermediatetransfer belt 27, and then causes the sensor 41 to detect the tonerpatches P while controlling the sensor 41 at the patch detection LEDlight amount level.

(b) The CPU 51 identifies a reflection light output from each of desiredportions of the intermediate transfer belt 27 based on the results ofthe above-mentioned two types of detection by the sensor 41. In thepresent embodiment, the CPU 51 sets the desired portion as a locationwhere a toner patch P is formed, and identifies the toner patchbackground reflection light output R(i).

(c) The CPU 51 calculates the density of each toner patch P, using thetoner patch reflection light output P(i) and the toner patch backgroundreflection light output R(i).

(d) The CPU 51 generates correction conditions based on the calculatedtoner patch P density, and corrects input image data according to thecorrection conditions.

These steps (a) to (d) will be described in detail. First, in the step(a), the CPU 51 causes the intermediate transfer belt 27 to perform onerotation with no toner patch P formed thereon, and causes the sensor 41to read the surface conditions of the intermediate transfer belt 27corresponding to one rotation of the same, so as to obtain theone-rotation profile of the intermediate transfer belt 27.

The CPU 51 stores data obtained from the sensor 41 at this time in theRAM 53, as the aforementioned second one-rotation profile of theintermediate transfer belt 27. In the image forming apparatus of thepresent embodiment, the rotational speed of the intermediate transferbelt 27 is set to 246 mm/sec, the circumferential length to 895 mm, andthe detection interval of the sensor 41 to 4 msec (the number of timesof detection per unit time is set to 250 times/sec). Therefore, 910 dataitems are obtained from the sensor 41 as shown by the following equation(1):

895(mm)÷246(mm/sec)÷( 4/1000(sec))≈910  (1)

More specifically, as shown in FIGS. 8A and 8B, the one-rotation profileis formed by a continuous sequence of 910 data items. The horizontalaxis in FIG. 8B represents data numbers n associated with the respectivedata items. As described hereinbefore, the two kinds of one-rotationprofiles are stored. One of them is the first one-rotation profileobtained by controlling the sensor 41 at the patch-detecting lightamount level and stored in the RAM 53, and the other is the secondone-rotation profile obtained by controlling the sensor 41 at thepattern-matching light amount level and stored in the RAM 53.

During the first rotation of the intermediate transfer belt 27, thesensor 41 is controlled to operate at the patch-detecting light amountlevel, and data from the sensor 41 is stored as the first one-rotationprofile. During the second rotation of the intermediate transfer belt27, the sensor 41 is controlled to operate at the pattern-matching lightamount level, and data from the sensor 41 is stored as the secondone-rotation profile.

Next, a description will be given of the reference profile of a portionof the intermediate transfer belt 27. In order to obtain the referenceprofile, the CPU 51 causes the sensor 41 to read the surface conditionsof the portion of the intermediate transfer belt 27 and then causes theRAM 53 to store the reflection light output from the sensor 41. At thistime, the sensor 41 is being controlled to operate at theabove-mentioned pattern matching LED light amount level.

The CPU 51 causes the sensor 41 to detect an area, where no toner imageis formed, between an image formed on a first recording sheet andanother image formed on a second recording sheet, or annon-image-forming area, such as between sheets, so as to obtain thereference profile. In a case where continuous printing is performed,space exists in a portion of the intermediate transfer belt 27corresponding to the area between the first recording sheet and thesecond recording sheet succeeding the first recording sheet. No image isformed in this space, and hence the surface of the intermediate transferbelt 27 is exposed. The sensor 41 reads reflected light from the space(specific portion) between the page images (toner images).

In the illustrated example, the sensor 41 irradiates light onto aportion of the intermediate transfer belt 27 between a page image J99for a 99th sheet and a page image J100 for a 100th page to therebydetect reflected light from the portion of the intermediate transferbelt 27. FIGS. 10A and 10B are diagrams showing the reference profile,in which FIG. 10A is a table of the reference profile, and FIG. 10B is agraph thereof. The vertical axis of the graph represents the sensoroutput from the sensor 41, and the horizontal axis represents thedetecting location (data number n) on the background. The graph in FIG.10B shows distribution of the reflection light output (sensor output)from the sensor 41, which is generated according to the reflected lightfrom the intermediate transfer belt 27.

In the present embodiment, it is assumed that a minimum length betweensheets in the image forming apparatus is set to 79 mm. The rotationalspeed of the intermediate transfer belt 27 is set to 246 mm/sec, and thedetection interval of the sensor 41 to 4 msec. Therefore, eighty dataitems are obtained from the sensor 41 as shown by the following equation(2):

79(mm)÷246(mm/sec)÷( 4/1000(sec))≈80  (2)

More specifically, the reference profile is formed by a continuoussequence of at least eighty data items. Detection for the secondone-rotation profile and detection for the reference profile areperformed by the same sensor 41, which means that the two profiles areobtained through detection of the same line in the direction of rotationof the intermediate transfer belt 27.

For this reason, unless the conditions of the intermediate transfer belt27 are changed e.g. by being scratched during a time period fromdetection of the second one-rotation profile to detection of thereference profile, the second one-rotation profile includes a data groupmatching with or closely analogous to the reference profile.

In the image forming apparatus of the present embodiment, the CPU 51performs pattern matching between the second one-rotation profile andthe reference profile so as to identify a correspondence between the twodata groups.

Next, a description will be given of a method executed in the step (b)for identifying the toner patch background reflection light output R(i)based on the results of detection by the sensor 41. As describedhereinabove, in the image forming apparatus of the present embodiment,the CPU 51 performs pattern matching between the second one-rotationprofile and the reference profile to thereby identify a data groupincluded in the second one-rotation profile and matching with thereference profile.

Further, based on the positional relationship on the intermediatetransfer belt 27 between the identified data group and a location wherethe toner patch P is formed, and the first one-rotation profile, the CPU51 identifies the toner patch background reflection light output R(i).This method will be described in detail.

The pattern matching is performed by determining a correlation functionbetween the second one-rotation profile and the reference profile.

As for correlation between discrete data groups Xi and Yi, as the valueof a correlation coefficient S(i) between the two data groups is closerto a value of 1, the correlation between Xi and Yi is higher, and thesimilarity therebetween is also higher. The correlation coefficient S(i)between the two discrete data groups Xi and Yi (i=0 to N−1) eachconsisting of N data items can be obtained by the following equation(3):

$\begin{matrix}{S = \frac{\sum\limits_{i = 0}^{N - 1}{\left( {{Xi} - {Xave}} \right)\left( {{Yi} - {Yave}} \right)}}{\sqrt{\sum\limits_{i = 0}^{N - 1}\left( {{Xi} - {Xave}} \right)^{2}}\sqrt{\sum\limits_{i = 0}^{N - 1}\left( {{Yi} - {Yave}} \right)^{2}}}} & (3)\end{matrix}$

In the present embodiment, Xi represents each of a continuous sequenceof eighty data items extracted from the second one-rotation profileformed by 910 data items. Xave represents an average value of theextracted eighty data items. Yi represents each of a continuous sequenceof eighty data items forming the reference profile. Yave represents anaverage value of these eighty data items.

More specifically, when the data group forming the second one-rotationprofile is formed by the data items X(i) (i=0 to 909), the CPU 51extracts a data group formed by a continuous sequence of eighty dataitems (e.g. data items X(0) to X(79)) from the 910 data items X(i).

A correlation coefficient S(0) is calculated by the following equation(4), based on a data group Y(j) (j=0 to 79) forming the referenceprofile and the data group X(i) (i=0 to 79) extracted from the secondone-rotation profile:

$\begin{matrix}{{S(0)} = \frac{\sum\limits_{i = 0}^{79}{\sum\limits_{j = 0}^{79}{\left( {{X(i)} - {Xave}} \right)\left( {{Y(j)} - {Yave}} \right)}}}{\sqrt{\sum\limits_{i = 0}^{79}\left( {{X(i)} - {Xave}} \right)^{2}}\sqrt{\sum\limits_{j = 0}^{79}\left( {{Y(j)} - {Yave}} \right)^{2}}}} & (4)\end{matrix}$

Similarly, function coefficients S(i) (i=0 to 909) between each of thedata groups forming the second one-rotation profile and the referenceprofile are calculated by the following equations (5) to (7):

$\begin{matrix}{{S(1)} = \frac{\sum\limits_{i = 1}^{80}{\sum\limits_{j = 0}^{79}{\left( {{X(i)} - {Xave}} \right)\left( {{Y(j)} - {Yave}} \right)}}}{\sqrt{\sum\limits_{i = 1}^{80}\left( {{X(i)} - {Xave}} \right)^{2}}\sqrt{\sum\limits_{j = 0}^{79}\left( {{Y(j)} - {Yave}} \right)^{2}}}} & (5) \\{{S(2)} = \frac{\sum\limits_{i = 2}^{81}{\sum\limits_{j = 0}^{79}{\left( {{X(i)} - {Xave}} \right)\left( {{Y(j)} - {Yave}} \right)}}}{\sqrt{\sum\limits_{i = 2}^{81}\left( {{X(i)} - {Xave}} \right)^{2}}\sqrt{\sum\limits_{j = 0}^{79}\left( {{Y(j)} - {Yave}} \right)^{2}}}} & (6) \\{{S(910)} = \frac{\sum\limits_{i = 910}^{910 + 79}{\sum\limits_{j = 0}^{79}{\left( {{X(i)} - {Xave}} \right)\left( {{Y(j)} - {Yave}} \right)}}}{\sqrt{\sum\limits_{i = 910}^{910 + 79}\left( {{X(i)} - {Xave}} \right)^{2}}\sqrt{\sum\limits_{j = 0}^{79}\left( {{Y(j)} - {Yave}} \right)^{2}}}} & (7)\end{matrix}$

The intermediate transfer belt 27 is an endless belt, and therefore inthe case of calculating the function coefficients S(832) to S(910), someof the eighty data items extracted from the data group X(i) arerepetitions from the start of the data group X(i). For example, a datagroup extracted so as to obtain the function coefficient S(831) isformed a total of eighty data items consisting of seventy-nine dataitems X(831) to X(909) and X(0). Further, a data group extracted so asto obtain the function coefficient S(909) is a total of eighty dataitems consisting of seventy-nine data items X(909) and X(0) to X(78). Itshould be noted that as for an expression “910+79” in the equation (7),X(910) corresponds to X(0), X(911) corresponds to X(1), and X(988)corresponds to X(78).

As described hereinbefore, as the value of the correlation coefficientS(i) is closer to the value of 1, the correlation between Xi and Yj ishigher, and the similarity therebetween is also higher. In this case,that the similarity is high means that there is a substantial matchbetween the pattern of a data group extracted from the secondone-rotation profile and that of the reference profile.

The image forming apparatus of the present embodiment determines that adata group extracted from the second one-rotation profile and having acorrelation function S(i) closer to 1 than any other correlationfunction S(i) (i=0 to 909) has a highest similarity to the referenceprofile. In short, the CPU 51 determines that the data group extractedfrom the second one-rotation profile and having a correlation functionS(i) closest to 1 is identical in location to the reference profile.

Thus, the CPU 51 sets the location of the portion of the secondone-rotation profile, which has the pattern matching that of thereference profile, as a reference position. The CPU 51 identifiesbackground data based on the positional relationship between thereference position and a location where a toner patch P is formed andthe first one-rotation profile.

First, the CPU 51 determines the data number n of a data itemcorresponding to a start of the reference position. Assuming that thisdata item is X(n) (0≦n≦909), the data item X(n) corresponds to theleading data item Y(n) of the reference profile. A toner patch P startsto be formed when T seconds have elapsed after detection of the dataitem Y(n). More specifically, the toner patch P starts to be formed froma location spaced by a predetermined distance from a location where thedata item Y(n) is detected. In other words, the toner patch P starts tobe formed from a predetermined location (second circumferentiallocation) determined with reference to a location (first circumferentiallocation in the circumferential direction of the intermediate transferbelt) where the data item Y(n) is detected.

FIG. 9 shows a method of identifying a toner patch background reflectionlight output R(i). The horizontal axis in the FIG. 9 graph representseach data number denoted by n of the data item X(n). The data number nis within a range of 0≦n≦909 as mentioned hereinbefore, and thereforethe maximum value of the horizontal axis is 909.

The timer 55 is turned on in synchronism with the start of detection ofspecular reflection light of the reference profile, and measures a timeperiod before the reading of the toner patch P (see FIG. 9) is to bestarted. The CPU 51 identifies the toner patch background reflectionlight output R(i), based on the result of measurement by the timer 55,the number of times of detection per unit time by the sensor 41, and thefirst one-rotation profile.

For example, it is assumed that the reflection light output from theleading portion of the reference profile is represented by X(n), andreading of the toner patches P is started T seconds after the timer 55starts the measurement. The image forming apparatus according to thepresent invention starts acquiring patch data by the sensor 41 when thetime measured by the timer 55 becomes equal to T seconds, i.e. slightlybefore the toner patch P is reached, and recognizes a locationcorresponding to several samples after a sampling point where the patchdata (sensor output) changes across a threshold value, as a leading endof the toner patch P. The leading end of the toner patch P is thusdetected based on the patch data detected by the sensor 41 because thetoner patch P does not always reach the reading position of the sensor41 accurately at theoretically expected time due to variations in therotational speed of the photosensitive drum 22 and the rotational speedof the intermediate transfer belt 27. Now, assuming that the leadingreflection light output R(i) from the background of the toner patch P isrepresented by X(m), since the detection interval of the sensor 41 isset to 4 msec, the number m of times of detection can be expressed bythe following equation (8):

m=n+1000T/4  (8)

Therefore, the leading reflection light output X(m) from the backgroundof the toner patch P can be calculated by the following equation (9):

X(m)=X((n+1000T/4)mod 910  (9)

A general expression of “A mod B” corresponding to a portion in theequation (9) represents the remainder of an integer A divided by aninteger B as a modulus (i.e. a remainder obtained by dividing theinteger A by the integer B). Since the intermediate transfer belt 27 isan endless belt, as mentioned hereinbefore, a toner patch P can beformed at a location between X(909) and X(0). This possibility is takeninto account in the equation (9).

One toner patch P is detected ten times at time intervals of 4 msec.Therefore, the reflection light output from the background of the tonerpatch P is stored as ten data items X((n+1000T/4)mod 910) toX(((n+1000T/4)mod 910)+9).

Thereafter, the toner patch background reflection light output R(i)formed by the ten data items is used for calculation of the density ofthe toner patch P.

Then, in the step (c), the CPU 51 calculates the density of the tonerpatch P using the toner patch reflection light output P(i) and the tonerpatch background reflection light output R(i). In the presentembodiment, the CPU 51 divides the toner patch reflection light outputP(i) by the toner patch background reflection light output R(i) tothereby calculate the density of the toner patch P. Specifically, theCPU 51 calculates the toner patch density DENS(i), i.e. the density of atoner patch P by the following equation (10):

DENS(i)=P(i)/R(i)  (10)

In this equation, R(i) is dependent on the surface conditions of aportion of the intermediate transfer belt 27 immediately under a tonerpatch P, and hence it can be calculated by the following equation (11):

R(i)=X((n+1000T/4)mod 910).  (11)

Therefore, the toner patch density DENS(i) is calculated by thefollowing equation (12):

$\begin{matrix}{{{DENS}(i)} = \frac{P(i)}{X\left( {\left( {n + \frac{1000T}{4}} \right){mod}\; 910} \right)}} & (12)\end{matrix}$

In the present embodiment, the sensor 41 detects each toner patch Phaving the same density ten times, so that the average value of theobtained ten data items is stored as the result of the detection of thetoner patch P. The average value of densities DENS(i) to DENS(i+9) isadopted as a final toner patch P density DENS_AVE.

Thus, the CPU 51 calculates the toner density. Since the density of atoner patch P is obtained using the equation (12) while taking intoaccount unevenness in the surface conditions of the intermediatetransfer belt 27, it is possible to accurately calculate the tonerdensity by the above-described correction method.

The degree of influence of unevenness in the surface conditions of theintermediate transfer belt 27 on a toner patch P depends on tonerdensity thereof. FIG. 11 is a graph showing a reflected light amountdistribution corresponding to one rotation of the intermediate transferbelt and a reflected light amount distribution in a case where tonerpatches are placed on the intermediate transfer belt, which areillustrated with the phases of the two distributions aligned with eachother. In areas p, q, and r in this graph, there are shown reflectedlight amount distributions in respective cases where the opticaldensities of the toner patches P (patch portions) are equal to “0.5”,“1.0”, and “1.5”, respectively.

As is understood from FIG. 11, it can be confirmed that in thelow-density patch area (area p), uneven gloss on the surface of theintermediate transfer member is reflected in the toner patch reflectedlight amount. In the high-density patch area (area r) where toner isdense enough to cover the background, uneven gloss on the surface of theintermediate transfer member is not reflected in the reflected lightamount. Thus, in the low-density patch area, the surface of theintermediate transfer member is partially exposed, and hence unevengloss is reflected in the reflected light amount.

For this reason, in the case of reading a plurality of patches rangingfrom a low-density patch to a high-density one, it is required to set athreshold value (D_TH) for the toner patch density DENS(i).

It is assumed that an average value of ten densities of a toner patch Pobtained using the equation (12) is represented by DENS_AVE. If theaverage value DENS_AVE is less than the threshold value D_TH, i.e.DENS_AVE≦D_TH, the CPU 51 calculates each of the densities DENS(i) ofthe toner patch P again, by an equation (13), referred to hereinbelow.More specifically, the CPU 51 calculates each of the toner patchdensities DENS(i) again as a reflection light output from theintermediate transfer belt as a background, using a reflection lightoutput R (one-rotation average) as the average of the values of thereflection light output from the intermediate transfer belt detectedduring one rotation of the intermediate transfer belt. Then, the CPU 51stores the average value of the ten toner patch densities DENS(i) as theresult of detection of the toner patch P.

DENS(i)=P(i)÷R(one-rotation average)  (13)

On the other hand, when DENS_AVE>D_TH, the CPU 51 does not calculateeach toner patch density DENS(i) again using the equation (13). Thus, inthe low-density patch area, it is possible to suppress reflection ofuneven gloss on the surface of the intermediate transfer member in thetoner patch reflected light amount.

The threshold value D_TH is different depending on a screen which isformed of dots regularly arranged in horizontal and vertical directions,for expressing shades of colors. More specifically, an image signallevel which causes the surface of the intermediate transfer member to bepartially exposed from a patch is different depending on the screen. Inthe present embodiment, the threshold value D_TH is set to 0.5.

Next, a description will be given of a method executed in the step (d),in which correction data is generated based on the calculated tonerpatch density and image data is corrected. The output gamma correctionunit 509 corrects the image data using the correction data.

First, a description will be given of the one-dimensional LUT as thecorrection data updated based on the results of detection of the tonerpatch densities. Here, only gradation correction of cyan color isdescribed, but correction of each of magenta, yellow, or black isperformed by the same method.

FIG. 12 is a graph showing the one-dimensional LUT stored in the RAM 53.The one-dimensional LUT is correction data for correcting input imagedata so as to make linear the relationship between the density of inputimage data and that of output image data. In FIG. 12, the horizontalaxis represents the density of input image data, and the vertical axisrepresents values of the toner patch density detected by the sensor 41.

Further, in FIG. 12, a straight line TARGET represents target gradationcharacteristics in the image density control of the present embodiment.

Points C1, C2, C3, C4, C5, C6, C7, and C8 correspond to detected valuesof the respective cyan toner patches P, and the curve γ represents adetected values of each toner patch density. Here, the curve γrepresents gradation characteristics in a state before execution of theimage density control. A gradation density for which a toner patch isnot formed in the curve γ is calculated by performing splineinterpolation such that the curve γ passes the origin of the graph andthe points C1 to C8.

A curve D represents a one-dimensional LUT calculated in the imagedensity control. The curve D is calculated by obtaining symmetricalpoints to the curve γ before correction with respect to the targetgradation characteristics TARGET. By correcting a detected density valuebased on the curve D, i.e. by multiplying the density of an input imageby a value on the curve D, for example, the gradation characteristics ofthe density of an output image corresponding to that of the input imagecan be made closer to the target gradation characteristics TARGET.

The calculated (generated) one-dimensional LUT (curve D) is stored inthe RAM 53 by replacing the existing one-dimensional LUT generated on apreceding occasion, whereby the update of the one-dimensional LUT iscompleted. Hereafter, the image forming apparatus corrects input imagedata based on the updated one-dimensional LUT and then forms an imagebased on the corrected image data, whereby images each formed withtarget densities can be obtained.

Next, a description will be given of the image density control executedby the image forming apparatus. FIGS. 13 and 14 are a flowchart of animage density control process. A control program implementing the imagedensity control process is stored in the ROM 52 and is executed by theCPU 51.

When the power of the printer is turned on, the CPU 51 causes theintermediate transfer belt 27 to perform one rotation without carryingtoner thereon and causes the sensor 41 to detect specular reflectionlight from the surface of the intermediate transfer belt 27 during theone rotation, while causing the sensor 41 to operate at thepatch-detecting light amount level. The results of the reading are sentto the RAM 53 and are stored therein as the first one-rotation profile(step S1).

Then, the CPU 51 causes the intermediate transfer belt 27 to perform onemore rotation without carrying toner thereon and causes the sensor 41 todetect specular reflection light from the surface of the intermediatetransfer belt 27 during the one rotation, while causing the sensor 41 tooperate at the pattern-matching light amount level this time. Theresults of the reading are sent to the RAM 53 and are stored therein asthe second one-rotation profile (step S2).

After execution of the step S2, the CPU 51 starts a job in response touser input of electronic data to the printer (job start). When the jobis started, the CPU 51 starts counting printed sheets (step S3).

The CPU 51 increases a counter value C of a sheet counter according tothe number of the printed sheets (step S4). Then, the CPU 51 determineswhether or not the counter value C is equal to a predetermined value(step S5). In the present embodiment, the toner patches P are formed intiming in which the number of printed sheets reaches 100. For thisreason, the predetermined value is set to a value of 99. Morespecifically, in the step S5, the CPU 51 determines whether or not thecurrent job is performing image formation on a 99th sheet.

If it is determined in the step S5 that the current job is notperforming image formation on the 99th sheet, the CPU 51 causes theprinter to execute a next job for image formation (step S6). On theother hand, if it is determined in the step S5 that the current job isperforming image formation on the 99th sheet, the CPU 51 causes thesensor 41 to detect a portion of the surface of the image carrierbetween page images immediately after completion of the job for the 99thsheet (step S7).

At this time, the sensor 41 is controlled to operate at theabove-mentioned pattern matching LED light amount level. Further, theCPU 51 turns on the timer 55 upon the start of detection to start timemeasurement.

The CPU 51 sends the results of the detection performed by the sensor 41in the step S7 to the RAM 53, and the data is stored as the referenceprofile in the RAM 53 (step S8).

The CPU 51 calculates a plurality of correlation coefficients based onthe above-mentioned equations (4) to (7) so as to derive a correlationbetween the second one-rotation profile and the reference profile, tothereby perform pattern matching between the second one-rotation profileand the reference profile (step 89). In the present embodiment, 910correlation coefficients are calculated. The CPU 51 identifies a datanumber corresponding to a data item indicative of a leading portion ofreflection light output from the reference profile, based on the resultsof the pattern matching (step S10).

After an image for the 99th sheet has been formed, the CPU 51 causes theimage forming units (toner image forming units) to form toner patches Pon the intermediate transfer belt 27 (step S11). With reference to theidentified data number, the toner patches P start to be formed from aportion of the intermediate transfer belt 27, which reaches thedetection position of the sensor 41 T seconds after turn-on of the timer55 in the step S7.

The CPU 51 identifies the background data of the toner patches P basedon information indicative of the location of data in the secondone-rotation profile corresponding to the reference profile, e.g. theaforementioned identified data number, and respective locations wherewhich the toner patches P are formed, e.g. data numbers associatedtherewith (step S12). The processing executed in the steps S9 to S12corresponds to the function of a circumferential location identificationunit.

The CPU 51 calculates the toner patch density DENS_AVE using the tonerpatch background data identified in the step S12 and detected data ofthe toner patches measured by the sensor 41 (step S13). The densitycalculation method is the same as described hereinbefore.

The CPU 51 generates a one-dimensional lookup table (LUT) for imageprocessing based on the calculated toner patch density DENS_AVE tothereby update the one-dimensional LUT stored in the RAM 53 (step S14).

Thereafter, the CPU 51 determines whether or not the job has beencompleted (step S15). If the job has not been completed, i.e. if theimage forming operation is to be continued, the CPU 51 resets the sheetcounter (step S16), followed by the process returning to the step S3. Onthe other hand, if the job has been completed, the CPU 51 brings theprinter into a standby state (step S17), followed by terminating thepresent process.

As described above, according to the image forming apparatus of thepresent embodiment, it is possible to identify the amount of reflectedlight from any location on the intermediate transfer belt in a shorttime by a simplified construction. Thus, the amount of reflected lightfrom the intermediate transfer belt as the background of toner patchescan be acquired in a short time.

Further, in the case of detecting the amount of reflected light from theintermediate transfer belt corresponding to one rotation of theintermediate transfer belt so as to perform pattern matching, the amountof light to be emitted from the light emitter and/or output gain of thelight receiver are/is increased, so that it is possible to prevent aplurality of candidate phases from being provided.

Furthermore, since reflection light output from a space between pageimages on the intermediate transfer belt is detected, it is possible toacquire the amount of reflected light from the intermediate transferbelt while forming an image, to thereby update the one-dimensional LUT(image forming conditions) in a short time.

It should be noted that the first and second one-rotation profiles inthe present embodiment are detected immediately after power-on of theimage forming apparatus and are stored in the RAM 53. Further, the CPU51 causes the intermediate transfer belt 27 to perform two rotationsafter completion of image forming operation for a predetermined numberof sheets, whereby the first and second one-rotation profiles aredetected again during the respective rotations to thereby update thefirst and second one-rotation profiles stored in the RAM 53. Thepredetermined number of sheets may be 1000 sheets, for example.

The surface of the intermediate transfer belt 27 wears due to contactwith a cleaning device for collecting remaining toner, and othermembers. For this reason, when images are repeatedly formed, gloss onthe surface of the intermediate transfer belt 27 increases. FIGS. 15Aand 15B are graphs showing the surface conditions of the intermediatetransfer belt 27 which vary with the cumulative number of printedsheets. FIG. 15A shows how the surface gloss changes, and FIG. 15B showshow sensor output changes. To cope with the aging of the surface of theintermediate transfer belt 27, the image forming apparatus of thepresent embodiment performs processing for detecting the first andsecond one-rotation profiles again depending on the cumulative number ofprinted sheets (recorded sheets).

Further, the image forming apparatus of the present embodiment isconfigured to detect the reference profile immediately after completionof image forming operation for a 99th sheet. However, the referenceprofile may be detected not only immediately after completion of theimage forming operation for the 99th sheet, but also before the imageforming operation for the 99th sheet, or pattern matching may beperformed between a plurality of reference profiles and the secondone-rotation profile.

In a case where pattern matching is performed between a single referenceprofile and the second one-rotation profile, a plurality of patternmatching areas can be identified. More specifically, a plurality ofcorrelation coefficients close to a value of 1 can be identified.However, by detecting from a plurality of areas a plurality of referenceprofiles associated therewith, respectively, and performing patternmatching using the plurality of reference profiles, it is possible toobtain more data groups for determining a correlation coefficient. Thismakes it possible to determine a more accurate correlation coefficientto thereby improve accuracy in pattern matching.

As described above, the method using pattern matching is effective as amethod of identifying the amount of reflected light from theintermediate transfer belt in a short time by a simplified construction.In the method using pattern matching, pattern matching is performedbetween the reflected light amount profile of a portion of theintermediate transfer belt exposed between sheets conveyed duringexecution of successive jobs (reference profile) and the reflected lightamount profile of the intermediate transfer belt corresponding to onerotation of the intermediate transfer belt (second one-rotationprofile), whereby the phase of the reference profile in the secondone-rotation profile is detected.

Conventionally, when the surface gloss of an intermediate transfer beltis relatively even and therefore the distribution of the reflected lightamount on the surface of the image carrier, which is detected by anoptical sensor, is relatively uniform, a plurality of candidate phasesare provided by pattern matching, which disables phase identification.In the present embodiment, however, since reflected light is detected atthe pattern-matching light amount level which is different from thepatch-detecting light amount level, it is possible to prevent aplurality of candidate phases from being provided by pattern matching.

FIGS. 16A and 16B are views for comparison between the pattern matchingin the first embodiment and the conventional pattern matching. In theconventional pattern matching shown in FIG. 16A, the LED light amount issmall, and hence two or more candidate phases are provided by colormatching. On the other hand, in the pattern matching in the presentembodiment shown in FIG. 16B, since the LED light amount is large, onlyone candidate phase is provided. Thus, phase identification isfacilitated.

An image forming apparatus according to a second embodiment performspattern patching by a different method from the method executed in thefirst embodiment. The construction of the image forming apparatusaccording to the second embodiment is the same as that of the imageforming apparatus according to the first embodiment, and thereforedescription thereof is omitted. Further, a density calculation processand a density control process are also the same as those in the firstembodiment except for pattern matching, and therefore description of thedensity calculation process and the density control process is omitted.

In the pattern matching in the second embodiment, first, the CPU 51calculates the absolute value of the difference between each of eightydata items of a data group extracted from the second one-rotationprofile and the reference profile. Then, when the total sum of theabsolute values calculated on a data group is smaller than that on anyother data group, the CPU 51 determines that the pattern of the datagroup of which the calculated total sum is the smallest matches that ofthe reference profile.

The pattern matching will be described in detail. First, the CPU 51extracts a continuous sequence of eighty data items from the secondone-rotation profile formed by 910 data items. The CPU 51 calculates thedifferences between each of the extracted eighty continuous data itemsD(i) and the respective associated eighty data items d(i) of thereference profile. More specifically, when data items D(0) to D(79) areextracted, the CPU 51 calculates the absolute value of the differencebetween the data item D(0) and the data item d(0) corresponding to thedata item D(0). Similarly, the CPU 51 calculates the absolute value ofthe difference between the data item D(1) and the data item d(1)corresponding to the data item D(1).

Then, when eighty absolute values are thus calculated, the CPU 51determines the total sum of the calculated absolute values. The CPU 51continues this operation and determines an extracted data group of whichthe calculated total sum is the smallest as having a pattern matchingthat of the reference profile.

Thus, the pattern matching performed by the image forming apparatus ofthe second embodiment can provide the same advantageous effect asprovided in the first embodiment. The pattern matching method can bemodified in various manners to perform more accurate pattern matching.

It should be noted that the present invention is not limited to theabove-described embodiments, but can be modified in various mannersbased on the subject matter of the present invention, which should notbe excluded from within the scope of the present invention insofar asfunctions as recited in the appended claims or the functions performedby the construction of each of the above described embodiments can beachieved.

For example, one of the pattern matching methods in the respective firstand second embodiments may be selectively employed, or alternatively thetwo methods may be both employed. In the latter case, when resultsobtained by the two pattern matching methods do not coincide with eachother, pattern matching is performed again. In this case, since themultiple pattern matching methods are employed, it is possible toachieve more accurate pattern matching.

The image forming apparatus of the present invention is implemented byan electrophotographic image forming apparatus, and as the image formingapparatus of this type, there can be mentioned a regular printingapparatus, a facsimile machine having a printing function, or amultifunction peripheral (MFP) provided with a print function, a copyfunction, a scan function, and so forth.

Further, although in the above-described embodiments, theelectrophotographic image forming apparatus is implemented by a colorimage forming apparatus, the present invention may be applied to amonochrome image forming apparatus.

In the above-described embodiments, the image forming apparatus, whichuses the intermediate transfer member, sequentially transfers tonerimages in the respective colors onto the intermediate transfer member insuperimposed relation, and then transfers the full-color toner imagecarried by the intermediate transfer member onto a recording medium in asingle operation. However, the invention is not limited to this transfermethod, but the image forming apparatus may be configured to use arecording medium carrier and sequentially transfer toner images in therespective colors onto the recording medium carrier in superimposedrelation. The intermediate transfer member may be implemented not onlyby a belt, but also by a drum.

Further, the shapes and relative positions of the component partsdescribed in the above-described embodiments can be changed, as deemedappropriate, according to the arrangement of an apparatus to which thepresent invention is applied, and various conditions, and therefore itis to be understood that the present invention is by no means limited tothe disclosed exemplary embodiments.

Furthermore, a sheet is not particularly limited in respect of itsmaterial and shape, but a paper medium, an OHP sheet, a thick sheet, anda tab sheet may be used.

Aspects of the present invention can also be realized by a computer of asystem or apparatus (or devices such as a CPU or MPU) that reads out andexecutes a program recorded on a memory device to perform the functionsof the above-described embodiments, and by a method, the steps of whichare performed by a computer of a system or apparatus by, for example,reading out and executing a program recorded on a memory device toperform the functions of the above-described embodiments. For thispurpose, the program is provided to the computer for example via anetwork or from a recording medium of various types serving as thememory device (e.g., computer-readable medium).

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2009-156770, filed Jul. 1, 2009, which is hereby incorporated byreference herein in its entirety.

1. An image forming apparatus that transfers a toner image carried by animage carrier onto a sheet, comprising: a photo detector unit disposedin facing relation to the image carrier, for detecting reflected lightfrom the image carrier; a circumferential location identification unitadapted to perform pattern matching between amounts of reflected lightdetected by said photo detector unit during one rotation of the imagecarrier and an amount of reflected light detected by said photo detectorunit from a specific portion of the image carrier in a circumferentialdirection of the image carrier, to thereby identify a firstcircumferential location of the specific portion of the image carrier inthe circumferential direction of the image carrier and then identify asecond circumferential location of a detection toner image formed on theimage carrier based on the identified first circumferential location; adensity calculation unit adapted to calculate density of the detectiontoner image based on an amount of reflected light from the detectiontoner image, which is detected by said photo detector unit, and anamount of reflected light from a portion of the image carrier in thesecond circumferential location identified by said circumferentiallocation identification unit, out of amounts of reflected light from theimage carrier corresponding to one rotation of the image carrier, whichare detected by said photo detector unit; and a density control unitadapted to control density of a toner image to be formed on the imagecarrier, according to the density of the detection toner imagecalculated by said density calculation unit.
 2. The image formingapparatus according to claim 1, wherein said photo detector unitincludes a light emitter adapted to emit light to be irradiated onto theimage carrier and a light receiver adapted to receive reflected lightfrom the image carrier, and wherein the photo detector unit is adaptedto be operable, when the amounts of reflected light corresponding to onerotation of the image carrier are to be detected for the patternmatching, to increase at least one of an amount of light emitted fromthe light emitter and an output gain of the light receiver in comparisonwith a case where the amounts of reflected light from the image carriercorresponding to one rotation of the image carrier are detected forcalculation of the density of the detection toner image.
 3. The imageforming apparatus according to claim 1, wherein the specific portion ofthe image carrier is a portion of the image carrier exposed betweentoner images sequentially formed on the image carrier.
 4. The imageforming apparatus according to claim 1, wherein the specific portioncomprises a plurality of specific portions, and said circumferentiallocation identification unit performs the pattern matching betweenrespective amounts of reflected light from the plurality of specificportions detected by said photo detector unit and the amounts ofreflected light from the image carrier corresponding to one rotation ofthe image carrier, to thereby identify the first circumferentiallocation of the specific portion in the circumferential direction of theimage carrier.
 5. The image forming apparatus according to claim 1,wherein said circumferential location identification unit determines acorrelation coefficient between the amount of reflected light from thespecific portion and each of the amounts of reflected light from theimage carrier corresponding to one rotation of the image carrier, andperforms the pattern matching based on the correlation coefficient. 6.The image forming apparatus according to claim 1, wherein saidcircumferential location identification unit calculates a differencebetween the amount of reflected light from the specific portion and anamount of reflected light extracted from the amounts of reflected lightfrom the image carrier corresponding to one rotation of the imagecarrier, and performs the pattern matching such that the absolute valueof the difference becomes minimum.
 7. The image forming apparatusaccording to claim 1, wherein the amounts of reflected light from theimage carrier corresponding to one rotation of the image carrier aredetected by said photo detector unit when toner images corresponding toa predetermined number of sheets have been formed on the image carrier.8. The image forming apparatus according to claim 1, wherein aftercalculating the density of the detection toner image based on the amountof reflected light from the image carrier in the second circumferentiallocation, if it is determined that the density of the detection tonerimage is not higher than a threshold value, said density calculationunit calculates the density of the detection toner image again based onthe amounts of reflected light from the image carrier corresponding toone rotation of the image carrier.
 9. A density control method for animage forming apparatus that is adapted to transfer a toner imagecarried by an image carrier onto a sheet, and includes a photo detectorunit disposed in facing relation to the image carrier, comprising:detecting reflected light from the image carrier by the photo detectorunit; performing pattern matching between amounts of reflected lightdetected by the photo detector unit during one rotation of the imagecarrier and an amount of reflected light detected by the photo detectorunit from a specific portion of the image carrier in a circumferentialdirection of the image carrier, to thereby identify a firstcircumferential location of the specific portion of the image carrier inthe circumferential direction of the image carrier and then identify asecond circumferential location of a detection toner image formed on theimage carrier based on the identified first circumferential location;calculating density of the detection toner image based on an amount ofreflected light from the detection toner image, which is detected by thephoto detector unit, and an amount of reflected light from a portion ofthe image carrier in the identified second circumferential location, outof amounts of reflected light from the image carrier corresponding toone rotation of the image carrier, which are detected by the photodetector unit; and controlling density of a toner image to be formed onthe image carrier, according to the calculated density of the detectiontoner image.
 10. A non-transitory computer-readable storage mediumstoring a program which, on execution by a programmable image formingapparatus that is adapted to transfer a toner image carried by an imagecarrier onto a sheet, and includes a photo detector unit disposed infacing relation to the image carrier, causes the programmable imageforming apparatus to carry out a density control method comprising:detecting reflected light from the image carrier by the photo detectorunit; performing pattern matching between amounts of reflected lightdetected by the photo detector unit during one rotation of the imagecarrier and an amount of reflected light detected by the photo detectorunit from a specific portion of the image carrier in a circumferentialdirection of the image carrier, to thereby identify a firstcircumferential location of the specific portion of the image carrier inthe circumferential direction of the image carrier and then identify asecond circumferential location of a detection toner image formed on theimage carrier based on the identified first circumferential location;calculating density of the detection toner image based on an amount ofreflected light from the detection toner image, which is detected by thephoto detector unit, and an amount of reflected light from a portion ofthe image carrier in the identified second circumferential location, outof amounts of reflected light from the image carrier corresponding toone rotation of the image carrier, which are detected by the photodetector unit; and controlling density of a toner image to be formed onthe image carrier, according to the calculated density of the detectiontoner image.